The boiling water reactor industry has experienced extensive cracking of internal reactor components. The core spray line piping system is of particular interest as the pipe joints provide many possible leak paths.
In a typical core spray line, the piping enters the vessel through a safe end above the shroud and immediately goes into a T-section that divides the flow into two pipes. It will be appreciated that a safe end as used herein refers to a coupling pipe used to create a fluid path between two pipes fabricated of dissimilar metals. The coupling pipe is constructed of a material compatible with the pipe sections to be joined.
Within the core spray piping system, each pipe curves around the interior of the vessel until it reaches a downcomer, with one or more supports along the path. The downcomer extends down to the shroud where it enters through another safe end and terminates in a T-section located in the sparger at the top of the shroud.
There are two piping systems per unit that serve four half circle spray spargers in the shroud. In existing systems, the joints are typically welded and subject to cracking, thus creating a leak path for the fluids retained by the pipes.
If cracks are found in the core spray line, it may be more advantageous to replace the cracked pipe than to leave the pipe in place and attempt repairs of the cracks. The restraining devices or supports that fix the pipe into position within the nuclear reactor must be removed in order to remove the pipes.
FIG. 1 shows apparatus known in the prior art for securing a core spray pipe (20) adjacent a wall (22) within a nuclear reactor. The pipe (20) is positioned adjacent a pad (24) mounted to the wall (22) and is positioned between studs (26) and (28). Studs (26) and (28) are typically identical, and consequently the description herein of either one of the studs can be applied to the other stud. The studs include a proximate end (27) connected to wall (22) and a distal end (29), and are spaced from each other a lateral distance (d1) as measured between parallel centerlines (CL1) and (CL2). The pipe (20) is held in place adjacent the pad (24) by restraining bar (30). The distal end (29) of studs (26) and (28) include bolts (36) that extend through bar ends (31) and (32) into a threaded hole in a boss (37). The bolts (36) include a shank (42) and head (44), with the head's outermost dimensional envelope defined by the width of bolt face (46) and the height of the bolt corner (47).
In one arrangement, the bolts (36) are screwed into bosses (37) far enough to bring restraining bar (30) into contact with pipe (20). However, the bolts are not screwed in far enough to close the gaps or grooves (40) between bar ends (31) and (32) and bosses (37). Consequently, the restraining bar (30) is in position to hold pipe (20) adjacent wall (22), but does not press against the pipe so firmly that it prevents the pipe's movement in the lateral direction or along the pipes longitudinal axis. Moreover, if desired, a slight gap (not shown) can be provided between pipe (20) and pad (24) and or between pipe (20) and restraining bar (30), to allow pipe (20) some capability of movement back and forth between the pad (24) and restraining bar (30). Thus, this arrangement allows the pipe (20) to move in response to expansions and contractions of the pipe that can be expected as a result of varying temperature cycles.
After the bolts (36) are positioned as desired, tack welds (38) are used to secure the bolts to the restraining bar (30). In some cases, weld material will overflow (not shown) into gap or grooves (40) and bond the restraining bar (30) to the bosses (37), so that the bolts (36), restraining bar (30), and bosses (37) are welded together as a single unit.
In order to remove the core spray pipe (20) from the nuclear reactor, the restraining bar (30) must be removed, which ordinarily requires removal of bolts (36). Since these bolts are welded in place, they must be removed by a process that overcomes the weld without damaging the studs. The shank (42) of the bolt may remain welded in place even if it is cut flush with the boss (37), requiring a tedious and time consuming process for removal without damaging the threaded holes in the studs into which new bolts must be placed. If the threaded holes are damaged during the removal process, the time and expense involved in removing and reinstalling the restraining bar (30) will become even more complicated, expensive, and time consuming. The problems associated with the foregoing difficulties an further aggravated by the facts that the restraining bracket may be located underwater, and in an area where it is preferable to reduce work exposure time.
From the foregoing, it is seen that a need exists for a new type of restraining bracket, and a method for its installation, that provides a means for securing a core spray pipe to a wall in a nuclear reactor and which can reduce the time and cost involved with replacing core spray pipes or damaged restraining bars.